Enhanced Solar Energy Absorption by Internally-Mixed Black Carbon in Snow Grains

By Mark G. Flanner, Xiaohong Liu, Cheng Zhou, J. Penner, Chaoyi Jiao. 2012

159 citations recorded by the source corpus.

Abstract. Here we explore light absorption by snowpack containing black carbon (BC) particles residing within ice grains. Basic considerations of particle volumes and BC/snow mass concentrations show that there are generally 0.05–10 9 BC particles for each ice grain. This suggests that internal BC is likely distributed as multiple inclusions within ice grains, and thus the dynamic effective medium approximation (DEMA) (Chýlek and Srivastava, 1983) is a more appropriate optical representation for BC/ice composites than coated-sphere or standard mixing approximations. DEMA calculations show that the 460 nm absorption cross-section of BC/ice composites, normalized to the mass of BC, is typically enhanced by factors of 1.8–2.1 relative to interstitial BC. BC effective radius is the dominant cause of variation in this enhancement, compared with ice grain size and BC volume fraction. We apply two atmospheric aerosol models that simulate interstitial and within-hydrometeor BC lifecycles. Although only ~2% of the atmospheric BC burden is cloud-borne, 71–83% of the BC deposited to global snow and sea-ice surfaces occurs within hydrometeors. Key processes responsible for within-snow BC deposition are development of hydrophilic coatings on BC, activation of liquid droplets, and subsequent snow formation through riming or ice nucleation by other species and aggregation/accretion of ice particles. Applying deposition fields from these aerosol models in offline snow and sea-ice simulations, we calculate that 32–73% of BC in global surface snow resides within ice grains. This fraction is smaller than the within-hydrometeor deposition fraction because meltwater flux preferentially removes internal BC, while sublimation and freezing within snowpack expose internal BC. Incorporating the DEMA into a global climate model, we simulate increases in BC/snow radiative forcing of 43–86%, relative to scenarios that apply external optical properties to all BC. We show that snow metamorphism driven by diffusive vapor transfer likely proceeds too slowly to alter the mass of internal BC while it is radiatively active, but neglected processes like wind pumping and convection may play much larger roles. These results suggest that a large portion of BC in surface snowpack may reside within ice grains and increase BC/snow radiative forcing, although measurements to evaluate this are lacking. Finally, previous studies of BC/snow forcing that neglected this absorption enhancement are not necessarily biased low, because of application of absorption-enhancing sulfate coatings to hydrophilic BC, neglect of coincident absorption by dust in snow, and implicit treatment of cloud-borne BC resulting in longer-range transport.

Subfields: Atmospheric Science, Global and Planetary Change

View Paper: https://www.semanticscholar.org/paper/beb010afc0116aced3aa536432f54acf5dac3d33

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Citations in the corpus

Influence of grain shape on light penetration in snow (2013, unknown): “However little is known about it (Flanner et al., 2012).”
Effects of internal mixing and aggregate morphology on optical properties of black carbon using a discrete dipole approximation model (2012, background): “Compared to bare BC, the MAC of the mixtures increased because (i) BC is surrounded by NaCl, which has a larger refractive index than the surrounding bulk medium (air) (Bohren, 1986; Flanner et al., 2012) and (ii) light is internally reflected in the nonabsorbing NaCl (Fuller et al.”
The DOE E3SM Coupled Model Version 1: Overview and Evaluation at Standard Resolution (2019, methodology): “(2012), causing aerosol optical properties within the snowpack to vary as a function of snow grain size and aerosol/ice mixing state (Flanner et al., 2012).”
An Overview of the Atmospheric Component of the Energy Exascale Earth System Model (2019, background): “…and light‐absorbing particles (e.g., BC and dust) can be deposited on snow and ice surfaces over land and sea ice, and these treatments were harmonized to treat both external mixing and internal mixing (within hydrometeor) of BC and snow grains following Flanner et al. (2012) and Flanner (2013).”